Plasma processing method and plasma processing apparatus

ABSTRACT

A plasma processing method characterized by placing a work to be processed on a work electrode being installed in a vacuum vessel and repeating alternately the following first step and the following second step; the first step comprising carrying out ion irradiation by applying a negative voltage to said work electrode while controlling said voltage such that the voltage of said work electrode versus the potential of said plasma be maintained constant, whereby making the energy possessed by ion to be irradiated to said work to be processed to provide an energy dispersed state as desired; and the second step comprising irradiating electrons in said plasma to said work to be processed by applying a positive voltage to said work electrode. And, an apparatus suitable for practicing said plasma processing method.

THE FIELD TO WHICH THE INVENTION PERTAINS

The present invention relates to a plasma processing method capable ofpracticing plasma processing at the time of performing etching,resist-ashing or film-formation particularly for a semiconductor waferutilizing plasma and an apparatus suitable for practicing said plasmaprocessing method.

The term "plasma processing method" in this specification means aprocessing in which highly active radicals and ions produced byplasmanizing a specific material are contacted with a work for example,substrate etc.) to thereby perform processing such as etching, filmdeposition, sputtering, cleaning or ashing for the work. And, the term"processing apparatus" means an apparatus to be used for practicing saidplasma processing method.

BACKGROUND OF THE INVENTION

The conventional plasma processing apparatus comprises a plasmaprocessing chamber comprising a vacuum vessel having a raw material gasinlet opening and a discharge opening and a means for supplyingelectromagnetic wave or the like in order to supply energy for producingplasma of a raw material gas.

The plasma processing method to be practiced using such plasmaprocessing apparatus utilizes high energy of radicals or ions and isapplicable to various desired processes including etching and filmdeposition by selectively deciding processing conditions such as therespective densities of the radicals and ions, the temperature of awork, etc. A method of controlling ions with electric charges isemployed in order to efficiently carrying out those processes.

Specifically, there is known a method in which a medium capable ofimparting energy for producing plasma in order to provide a plasmastate, a high-frequency magnetic wave of 13.56 MHz is applied to theprocessing gas to produce plasma, by which an energy is provided for theions to reach the work, thereby performing the control of the ions. Thismethod can be practiced using an apparatus shown in FIG. 9. FIG. 9 is aview schematically illustrating an example of the reactive ion etchingapparatus (RIE) for etching a work by producing plasma by the action ofa high-frequency magnetic wave of 13.56 MHz. see, Applied Physics, vol.51, volume 3, pp. 350 (1982))

In the reactive ion etching apparatus shown in FIG. 9, a work electrode909 is placed in a vacuum vessel 903 through an insulator 910, and acounter electrode 920 is arranged while facing to the work electrode909. Plasma 906 is formed in a space between the work electrode 909 andthe counter electrode 920, and a work 908 on the work electrode 909 isetched by the action of the resulting plasma. Numeral reference 905stands for a processing gas inlet opening and numeral reference 923stands for a matching box.

Explanation is to be made about the case of etching a Si substrate usingCl₂ gas in the etching process using the above apparatus shown in FIG.9.

First, the work Si substrate 908 to be etched is positioned on the workelectrode 909 and the inside of the vacuum vessel 903 is evacuated by anexhausting system to a vacuum degree of less than 10⁻⁴ Torr for example.

Then, the Cl₂ gas as the etching gas is introduced into the vacuumvessel 903 through the gas inlet opening 905, and the inside of thevessel is maintained at 0.05 Torr for example. A high-frequency powerfrom a high-frequency power source 919 is applied through the matchingbox 923 to the work electrode 909 to thereby produce plasma 906 betweenthe work electrode and the counter electrode 920.

Herein, the work electrode 909 in contact with the plasma 906 iselectrically isolated from earth electric potential by way of theinsulator 910 and a condenser (not shown) in the matching box 923.Because of this, a negative bias voltage (this is also called a cathodedrop voltage; its degree is about a maximum value Vm of thehigh-frequency to have been applied) is induced at the work electrode909 due to the difference between the mobility of an electron and thatof an ion. And ions are accelerated with the action of the bias voltageto collide the work 908 together with radicals, whereby the work isetched. In this case, the ion energy is controlled by adjusting thehigh-frequency power to be applied and varying the bias voltage.

However, in the case of using this RIE apparatus, the ion energy toreach the surface of the work has an extent of about 2 eVm (the e hereinis the charge quantity of an electron) and because of this, damagecaused on the work due to ions having a high energy becomes seriouslyproblematic in the case where the surface state at the surface of thework is important. On the other hand, in the case of performing theetching while reducing the ion energy, that is, reducing thehigh-frequency voltage (reducing the high-frequency power) in order toprevent occurrence of such damage, there is caused a different problemthat the etching rate is reduced accordingly, wherein an appropriateetching rate cannot be attained as desired. Under these circumstances,there is an increased demand for provision of such a plasma processingmethod that excels in the controllability, does not cause damage at thesurface of a work and provides a high processing speed.

The above prior art is to perform etching by producing plasma with theapplication of high frequency energy to impart energy to ions. Otherthan this, there is known a plasma processing method in which microwaveis used in order to produce plasma and a high frequency power is appliedto a work electrode in order to control the energy of ion to beirradiated to a work. According to this processing method, the plasmaprocessing can be carried out with a good controllability since thecontrol of plasma state and the control of the energy of an ion to beimpinged to the work can be independently performed on each other. Anexample of the apparatus for this method is described in Japanese PatentPublication Sho.56(1981)-37311. This apparatus is of the constitutionwhich is schematically shown in FIG. 10.

In the case where the apparatus of FIG. 10 is used for etching process,microwave generated by a microwave generating device 123 (magnetron, forinstance) propagates in a waveguide 101 and is absorbed by an etchinggas (introduced through a leak valve 124) being controlled, for example,to less than 1 atmospheric pressure in a vessel 107 made of aninsulating material which is placed in a mirror field provided by amagnetic field-causing coil 104 and a permanent magnet 127.

In a processing chamber 125, there are provided a gas inlet opening 105,a gas discharge opening 126, a work electrode 109, a work 108, saidpermanent magnet 127, and an insulator 130. Active ions in plasma 106 asproduced are impinged into the work 108 along the mirror field tothereby etch the surface of the work 108. At this time, the work 108 andthe work electrode 109 are applied with an AC voltage of a highfrequency magnetic wave as shown in the figure. As its applicationmeans, there is a manner that a high frequency power source 119 isconnected to an upstream coil of an air-core transformer 128 of about1:1 in turn ratio, the work electrode 109 is connected to one end of adownstream coil, and a capacitor 129 is connected to the other end ofsaid downstream coil. And the other end of the capacitor 129 iselectrically grounded. The circumferential wall of the processingchamber 125 is provided with the insulator 130 to electrically float thework electrode 109. The work electrode 109 is made of an electricallyconductive material and because of this, the high frequency voltageapplied to the work electrode 109 is applied also to the workconcurrently. The capacitor 129 has a capacity of about 0.1 μF forexample, and it functions to make the work 108 isolated from the earthin a direct current-like manner and to allow only a high frequencycurrent to pass through the capacitor.

Therefore, the situation becomes such that a high frequency voltagenegatively biased by Vf as Vs shown in FIG. 11 is applied to the workelectrode 109. Ions (the ions mentioned hereinafter mean positive ions;and they are mostly monovalent) which arrive at the work (or the workelectrode 109) or electrons are accelerated or decelerated due to anelectric potential difference Vs--Vp with the plasma potential Vp. Themean bias value Vf is decided so that the quantity of arrival electriccharges of those ions becomes equal to that of those electrons in termsof time average. The Vp herein is of a value of 10 to 20 V. As for itswave form, shown in FIG. 11 is a sine wave in terms of expediency. Inpractice, the wave form is somewhat varied due to nonlinear effects ofthe plasma.

Here, the ion energy arriving at the work 108 has an energy of e(Vf-Vp)(e herein is a charge quantity of the electron) on the averageand Vp becomes greatly smaller than Vf (Vf) Vp). Because of this, theion energy arriving at the work can be approximated to be about eVf.Ions collide the work with this average energy, whereby the work isdesirably processed. The value of this energy can be properly controlledby the Vs, particularly, the power to be outputted by the high frequencypower source. Such control makes it possible to conduct processing witha good controllability or to conduct high speed processing with ionshaving a high energy.

By the way, in general, in the case of subjecting a wafer of Si or SiO₂to plasma processing with the use of an ion energy, there is a slightoccasion for the work to be damaged as long as ions having an energy ofless than 100 eV are used. However, the processing speed is heightenedas the energy increases, and because of this, the energy width of an ionto be used for conducting the processing at a high speed without causingdamage for the work is required to be narrow. The situation for thisenergy width to be narrowed is schematically shown in FIG. 12. Thefigure illustrates the situation where the number of ions having anenergy of 100 eV or nearby this is markedly large and on the other hand,the number of other ions having an energy of other value than that inthe former is markedly small. And, in the case where starting at theborder of certain energy for ions, deposition comes to cause in a regionwhere the energy is low and etching comes to cause in a region where theenergy is high, use of ions having an energy with a narrow energy widthmakes it easier to control the processing. FIG. 13 illustrates thesituation where deposition or etching is caused depending upon themagnitude of the energy of an ion to be used. However, in the prior art,because the voltage to be applied to the work is varied as shown in FIG.11, the energies of ions to be impinged into the work are distributed upto 2e at the maximum Vo--Vpo)(where Vo and Vpo are maximum values of theVp and Vs shown in the figure). For instance, when a high frequencyvoltage of Vo=100 V is applied, there will be caused ions having anenergy of 100 eV on the average and about 200 eV at the maximum.Therefore, the energy width for the ions is eventually spread. Becauseof this, there is a problem that it is impossible to constantly andperform plasma processing at a high speed and with high efficiency.

SUMMARY OF THE INVENTION

The present invention makes it a principal object to overcome theforegoing problems and to provide a plasma processing method whichenables to perform plasma processing at a high speed and with a highefficiency without causing a damage for a work by using ions which havean energy value being controlled to desired value and which are small inenergy variance.

Another object of the present invention is to provide a plasmaprocessing method which enables to constantly perform plasma processingwith an excellent controllability.

A further object of the present invention is to provide an apparatussuitable for practicing the above plasma processing.

The present invention has been accomplished as a result of repeatedstudies by the present inventor in order to solve the foregoing problemsin the prior art and in order to attain the above objects.

The plasma processing method to be provided according to the presentinvention is of the constitution as will be described below.

That is, a plasma processing method comprising introducing a processinggas into a vacuum vessel containing a work to be treated being placed ona work electrode therein, applying a plasma generating energy to saidprocessing gas to cause plasma and treating the work to be treated withthe resultant plasma, characterized by repeating alternately thefollowing first and second steps, the first step: carrying out ionirradiation by applying a negative voltage to said work electrode whilecontrolling said voltage such that the voltage of said work electrodeversus the the potential of said plasma is maintained constant, wherebymaking ions having an energy to be irradiated from said plasma to saidwork to be treated to provide a dispersed state as desired with respectto their energies; and the second step: irradiating electrons in saidplasma to said work to be treated by applying a positive voltage to saidwork electrode.

The present invention includes a plasma processing apparatus suitablefor practicing the above plasma processing method. The plasma processingapparatus is of the constitution as will be described below.

That is, a plasma processing apparatus comprising a vacuum vessel havingan electrode on which a work to be treated can be positioned therein,said vacuum vessel being capable of confining plasma capable ofperforming plasma treatment for said work; gas supply means forsupplying a processing gas to be used for the treatment of said work tobe treated into said vacuum vessel; exhaust means for vacuum-evacuatingthe inside of said vacuum vessel; and voltage applying means forapplying a voltage to said work electrode and controlling ions in saidplasma to be impinged into said work to be treated, characterized inthat said voltage applying means is designed such that a positivevoltage and a negative voltage can be alternately applied and thevoltage of said work electrode versus the potential of said plasma canbe controlled to be constant when the voltage of said work electrodeversus the the potential of said plasma is negative.

According to the plasma processing method or the plasma processingapparatus respectively of the above-mentioned constitution according tothe present invention, desirable plasma processing can be performed at ahigh speed and with a high efficiency since energy distribution of ionsarriving at the surface of a work to be treated can be completed in adesired state and the value of energy of those ions can be properlycontrolled. Particularly, for instance, in the case of performingetching, extremely efficient etching can be performed by completingenergy of ions arriving at the surface of the work to be in a desiredstate suitable for etching. Likewise, in the case of performingfilm-formation, it is possible to form a deposited film excelling incharacteristics with a high efficiency by completing said energy of ionsto be of a high magnitude while not causing damage to the surface of thework.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1(A) is a schematic view illustrating an embodiment of the plasmaprocessing apparatus of the present invention. FIG. 1(B) is a circuitdiagram illustrating an embodiment of a circuit usable in the plasmaprocessing apparatus of the present apparatus.

FIG. 2 illustrates an example of the reference waveform outputted from areference waveform signal-outputting mechanism which is applicable as aconstituent of the plasma processing apparatus of the present invention.

FIG. 3 is a schematic view illustrating an embodiment of a controlmechanism for controlling the voltage of a work in the plasma processingapparatus of the present invention.

FIG. 4 is sa schematic view illustrating an embodiment of the plasmaprocessing apparatus of the present invention.

FIG. 5 is a graph showing the distribution of energy possessed by ionimpinged into the work electrode.

FIG. 6 is a schematic view illustrating an embodiment of theconventional plasma processing apparatus.

FIG. 7 is a graph showing the relationship between the t₁ and the t₂ ofa reference waveform outputted by a reference waveform signal-outputtingmechanism which is applicable as a constituent of the plasma processingapparatus of the present invention.

FIG. 8 is a schematic view illustrating an embodiment of the plasmaprocessing apparatus of the present invention.

FIG. 9 is a schematic view illustrating an embodiment of theconventional plasma processing apparatus.

FIG. 10 is a schematic view illustrating an embodiment of theconventional plasma processing apparatus.

FIG. 11 is a graph showing changes of a voltage applied to the workelectrode.

FIG. 12 is a graph showing the relationship between the ion energy andthe number of ions.

FIG. 13 is a graph showing the ion energy and the sputtering rate.

DETAILED EXPLANATION OF THE PREFERRED EMBODIMENTS

In the following, the present invention will be explained in more detailwhile referring to the drawings.

FIG. 1(A) is a schematic view illustrating a preferred embodiment of theplasma processing apparatus using microwave according to the presentinvention. In the figure, numeral reference 101 stands for a waveguidefor supplying microwave, numeral reference 102 stands for a microwavetransmitting window through which microwave transmits, numeral reference103 stands for a vacuum vessel made of a metal, numeral reference 104stands for an air-cored coil for providing a magnetic field in thevacuum vessel 103, and numeral reference 105 stands for a gas inletopening for introducing a processing gas into the vacuum vessel. Numeralreference 106 stands for plasma generated in the vacuum vessel, numeralreference 107 stands for an insulating vessel made of quartz, alumina,etc., numeral reference 108 stands for a work to be treated, numeralreference 109 stands for a work electrode made of a conductive materialon which the work 108 is to be positioned, and numeral reference 110stands for an insulator for electrically isolating the work electrode109 from earth-electric potential. Each of numeral references 111a and111b stands for a langmuir probe for measuring a potential of the plasma106, and each of numeral references 112a, 112b and 112c stands for anattenuator. Numeral reference 113 stands for a plasma potentialappreciation system for appreciating and computing a potential Vp of theplasma based on the voltage detected by the langmuir probes 111a and111b. Each of numeral references 150a and 150b stands for a DC powersource for setting an outer voltage for computing a plasma potential atthe position where the langmuir probe 150a or 150b is located. Numeralreference 114 stands for a system for appreciating the voltage betweenthe plasma and the work electrode which is capable of computing apotential Vsp of the work electrode 109 versus the plasma and outputtinga signal relating to the voltage of the work electrode. Numeralreference 115 stands for a reference waveform signal-outputting systemwhich serves to output a reference waveform signal having an idealwaveform to be applied to the work electrode 109. Numeral reference 116stands for a comparison and control signal-outputting system whichserves to compare the signal relating the voltage of the work electrodewith the reference waveform signal whereby detecting a differencebetween them and to output a signal relating to controlling the voltageof the work electrode. Numeral reference 117 stands for a controlsignal-amplifying system which serves to amplify the above signaloutputted. Numeral reference 118 stands for a work electrodevoltage-controlling system which serves to control the voltage of thework electrode using the signal amplified to be such that it has awaveform identical to that of the reference voltage and the value ofsaid waveform comes to have a desired value. Each of numeral references131 and 132 stands for a DC power source which serves to supply a DCpower to the work electrode voltage-controlling system 118.

In the plasma processing apparatus described above, the ideal waveformshould be properly set up depending upon the kind of the plasmaprocessing to be practiced. However, in general, a waveform having apositive potential and a negative potential is set up so that the workelectrode 109 is not in a state of being charged up.

In the following, explanation is to be made about an example of theprocess of the plasma processing using the plasma processing apparatushaving the foregoing constitution.

First, a processing gas (for example, CHF₃ or the like in the case ofetching SiO₂ ; SF₆, Cl₂, etc. in the case of etching Si; SiH₄ +N₂ +H₂ orthe like in the case of forming a SiN film; O₂, or the like in the caseof ashing a novolak type resist) is introduced into the vacuum vessel103 through the gas inlet opening 105. The vacuum vessel 103 is vacuumevacuated to bring its inside to a desired processing pressure (forexample, a vacuum of 5×10⁻⁴ to 1×10⁻¹ Torr) The inside of the vacuumvessel is maintained at this pressure. Then, a magnetic field is causedin the vacuum vessel 103 by actuating the air-cored coil 104.

Thereafter, microwave (of 2.45 GHz in usual case) from a magnetronoscillator (not shown in the figure) is forced to propagate through thewaveguide 101 and transmit through the microwave transmitting window 102being hermetically sealed between the vacuum vessel 103 and thewaveguide 101 into the insulating vessel 107.

In this way, there is caused plasma 106 in the vacuum vessel 103. Inthis case, when the magnitude of the magnetic field is made to be avalue (875 Gauss) at which an electron cyclotron resonance is caused tothe microwave of 2.45 GHz, the microwave is effectively absorbed toproduce highly dense plasma.

There are various methods for appreciating the plasma potential Vp. Theplasma processing apparatus of FIG. 1 is configured such that it can beappreciated by using the langmuir probes 111a and 111b. The langmuirprobes 111a and 111b herein are desired to be installed at the positionover the work electrode 109 while leaving between them an interval (forexample, about 0.5 mm to about 1 cm) of being several times to severaltens times over the length of an ion sheath caused between the plasmaand the work electrode 109. In this embodiment, it is so designed thatthe measurement is carried out by the two langmuir probes. However, itis desired to further increase the number of the langmuir probes inorder to improve the measuring accuracy.

A specific example of the method of appreciating the plasma voltage isto be explained.

That is, first, each of the langmuir probes 111a and 111b is made to beof a floating potential of the plasma 106 without externally applyingany voltage thereto. Herein, the respective values are assumed to beVpf-a and Vpf-b, respectively. Each of the floating potentials Vpf-a andVpf-b induced respectively at the langmuir probes 111a or 111b is madeto be a voltage suitable for successive appreciation treatment by theattenuator 112a or 112b. In this embodiment, it is possible to transformthe voltage inputted into these attenuators to a value of 1/10 thereof.The voltage thus transformed to 1/10 is inputted into the plasmapotential appreciating system 113.

Specifically, in the case where the pressure in the vacuum vessel 103 isless than 10⁻² Torr, collision among ions within the ion sheath isslightly caused, and because of this, from the Bohm condition, theplasma potential Vp can be expressed by the following equation (1):##EQU1##

In the above, k is a Boltzman's constant, Te is an electron temperature,and M_(i) is a mass of an ion.

Since the electron temperature Te is substantially not changed duringthe process, it is possible to measure the volt-ampere characteristic ofeach of the langmuir probes 111a and 111b and to determine each of theelectron temperatures Te-a and Te-b at each of the measuring positions.Particularly, as apparent from the equation (1), each of the plasmapotentials Vpa and Vpb at the respective measuring positions can beeasily obtained by detecting each of the floating potentials Vpf-a andVpf-b and detecting each of the electron temperatures Te-a and Te-b.

In more detail, the value of V_(T), that is, the value of each of V_(T)-a and V_(T) -b at the position where each of the langmuir probes 111aand 111b is situated is computed, and the value corresponding to, forexample, 1/10 of the resultant value in each case is inputted into theplasma potential appreciating system 113 by properly adjusting - each ofthe setting DC power sources 150a and 150b. Then, using an addingcircuit comprising an operation amplifier in the plasma appreciatingsystem 113 as the fundamental element, each of the plasma potentialsVp-a=V_(T) -a+Vpf-a and Vp-b=V_(T) -b+Vpf-b at the position where eachof the langmuir probes 111a and 111b is situated is calculated to obtaina plasma potential in each case, and the mean value of the Vp-a and theVp-b is obtained by an average-calculating circuit. This value has amagnitude corresponding to 1/10 of the actual value, and it is outputtedas a signal relating to the plasma potential.

The work electrode voltage Vps (versus earth potential) is transformedby the attenuator 112c at the same transformation ratio of 1/10 as inthe the above case and for the same reason as in the above case wherethe voltage of the floating potential Vpf of the plasma is transformedby using each of the attenuators 112a ad 112b. The work electrodevoltage Vps transformed to 1/10 is outputted as a signal relating to thework electrode voltage from the attenuator 112c.

In the appreciating system 114 for the voltage between the plasma andthe work electrode into which the signal relating to the plasmapotential and the signal relating to the work electrode voltage versusthe earth potential have been inputted, the plasma-work electrodevoltage signal is subtracted from the signal relating to the workelectrode voltage versus the earth potential by the calculating circuittherein to obtain a work electrode voltage Vps to the plasma, and theresultant is outputted as a signal relating to the plasma-work electrodevoltage. The value of Vps in this case is corresponding to 1/10 of theactual value.

Shown in FIG. 2 is of a signal V_(R) relating to the reference waveformwhich is outputted by the reference waveform signal-outputting system115. The reference waveform signal V_(R) is identical to the idealwaveform of the voltage between the plasma and the work. And its valuehas been transformed to a value of 1/10 for example.

In FIG. 2, t₁ stands for a period of time when an electron current isflown into the work electrode 109 from the plasma, t₂ stands for aperiod of time when an ion current is flown into the work electrode 109from the plasma 106, and each of V₁ and -V₂ stands for a voltagecorresponding to 1/10 of the voltage of the work electrode 109 versusthe plasma 106 during the respective period of time.

First, consideration is to be made about the period of time with respectto each of t₁ and t₂. During the period of time t₂, ions are flown intothe work electrode 109 to process the work 108. If nothing is done inthis case, the charges of the ions flown stay at the work electrode 109to heighten the voltage of the work electrode 109, whereas the number ofelectrons is increased in comparison with that of ions in the plasma 106and this results in decreasing the plasma potential, wherein ions stopflowing into the work electrode 109 shortly.

In this respect, during this period of time t₂, in order to maintain theenergy of the ions arriving at the work electrode 109 constant, thevoltage V_(PR) between the plasma and the work is maintained constantand control of making the energy received from the magnetic fieldbetween the plasma and the work constant is carried out as follows.

That is, the signal relating to the voltage between the plasma and thework electrode obtained the appreciating system 114 for the voltagebetween the plasma and the work electrode and the reference waveformsignal obtained from the reference waveform signal-outputting system 115are inputted into the control signal-outputting system 116, wherein asignal corresponding to a difference between the two inputted signals isoutputted by the action of a differential amplifier. The signaloutputted is then amplified by the control signal-amplifying system 117and inputted into the work electrode voltage-controlling system 118.

Shown in FIG. 1(B) is an example of the fundamental circuit of theforegoing work electrode voltage-controlling system to be used in thepresent invention. In FIG. 1(B), numeral reference 332 stands for annpn-type transistor, numeral reference 333 stands for a pnp-typetransistor, each of numeral references 334 and 335 stands for anelectric resistor for controlling a base current of each of saidtransistors, and each of numeral references 131 and 132 stands for a DCpower source.

In the work electrode voltage-controlling system having suchconstitution as mentioned above the transistors 332 and 333 arepositioned between the DC power source 131 and the DC power source 132.The voltage between the collector and the emitter (of the transistors332 and 333) is controlled by the current flowing through the bases ofthe transistors controlled by the control signal outputted from thecontrol signal-amplifying system 117, and as a result, the voltage Vpsto be applied to the work electrode 109 versus the plasma is maintainedat a constant value, for example, -10 V₂.

Since the voltage of the work electrode 109 is constant to the plasma106, the energy of ion arriving at the work can be completed to aconstant value, for example, -10 eV₂. Accordingly, plasma processing canbe performed by using ions of a narrow energy width. And, also withrespect to the energy of ion, appropriate precise control is possible byvarying the value V₂.

The length of t₂ is determined due to the control limit of the workelectrode voltage-controlling system 118. As for the t₂, it may be acertain period of time less than that. For instance, it may be made tobe a constant value of less than the period of 10 μsec. Other than this,the length of the t₂ may be determined by transmitting a control limitsignal from the work electrode voltage-controlling system 118 to thereference waveform signal-outputting system 115.

Next, consideration is to be made about the t₁. During the t₁, theplasma potential descended and the work electrode voltage raised (theearth potential is the reference in each of the two cases) during thet₂, are respectively raised and lowered. During the t₁, an electronhaving a high traveling speed is flown into the work electrode from theplasma 106, and because of this, the t₁ may be shorter than the t₂.

The range for the value of the t₁ when V₁ ≧0 and under the Bohmcondition is such as expressed by the following equation: ##EQU2##

Herein, when the mass of Mi is made to be about 10 in terms of massnumber (almost all the ions are of more than 10 in mass number), thesituation becomes:

    t.sub.l ≧1.1×10.sup.-2 t.sub.2                (3)

From the above, it is understood that the t₁ should satisfy the equation(3).

However, when the situstion stays at V₁ >0 for an excessive period oftime, both the plasma potential Vp and the work electrode voltage areexcessively lowered (the earth potential is the reference in each of thecases). Therefore, when either the plasma potential or the workelectrode voltage becomes to be a certain value (for example, a valueobtained when the work electrode voltage is not controlled, or the earthpotential), it is possible to terminate the t₁ by outputting a signal toterminate the t₁ from the attenuator 112c and transmitting the signal tothe reference waveform signal-outputting system 115. (see, FIG. 3)

In the case of using a signal to determine the above t₁ and t₂, aconstant voltage having a negative value should be caused at thereference voltage-outputting system 115. In this case, said constantvoltage may be of a reference voltage waveform not having such voltagepart with a positive value as shown in FIG. 2.

The waveform of the V_(R) during the period of time of the t₁ ismaintained constant without being changed during the period of time ofthe t₁ in the case of FIG. 2. However, during this period of time, it isnot necessary for the energy of electron to be maintained constant, andbecause of this, any waveform can be employed as long as the V₁ duringthis period of time satisfies the equation: V₁ ≧0.

In addition, in the the case where the insulating vessel 107 is notpresent, the potential of the plasma 106 is not largely changed sincethe potential of the plasma 106 is in contact with the vacuum vessel103, the plasma 106 in said vacuum vessel being maintained at the earthpotential.

In the case where the change in the potential of the plasma 106 isslight as described above it, is not substantially problematic even ifthe work electrode voltage is used instead of the plasma-work electrodevoltage. In view of this, it is possible to simplify the apparatus.

The plasma processing apparatus as explained above is for the case usingmicrowave. However, the apparatus according to the present invention maybe optionally structured t be of other constitution in which otherenergy than microwave is used. As an embodiment of such apparatus, thereis shown an apparatus using high frequency magnetic wave in FIG. 8.

In FIG. 8, numeral reference 803 stands for a vacuum vessel, numeralreference 805 stands for a gas inlet opening, numeral reference 806stands for plasma, numeral references 810a and 810b stand forrespectively an insulator, and numeral reference 819 stands for a highfrequency power source. Numeral reference 820 stands for an upperelectrode which is electrically isolated from the vacuum vessel 803 bythe insulator 810b and which is situated opposite to the work electrode109. Numeral reference 823 stands for a matching box, and numeralreference 821 stands for a switch. The same numeral references as thosein FIG. 1 are respectively of the same meaning as that expressed by thecorresponding numeral reference in FIG. 1.

In the case of using the apparatus of FIG. 8, a work 108 is firstpositioned on the work electrode 109, and the inside of the vacuumvessel 803 is sufficiently vacuum-evacuated. Then, a processing gas ofthe same kind as in the foregoing case is introduced through the gasinlet opening 805, and the pressure in the vacuum vessel is maintainedat a vacuum of, for example, 10⁻³ to 1 Torr. Successively, the switch821 is positioned on the side of the high frequency power source 819.And, the high frequency power source 819 having a specific frequency,for example, in the frequency range of 1 to 300 MHz is oscillated toapply a high frequency power to the upper electrode 820. In this way,plasma 806 is caused between the upper electrode 820 and the workelectrode 109. In this case, the matching box 823 is adjusted such thata reflecting electric power becomes minimum. Successive procedures afterthis are the same as those explained in the case of the apparatus ofFIG. 1.

The plasma processing apparatus of the present invention may take otherconstitution than those described above. An embodiment of suchconstitution is to be explained in the following.

That is, in the case where the switch 821 is positioned on the earthside and the upper electrode 820 is grounded, the situation comes to aresult that electrical connection is not present between the highfrequency power source 819 and the upper electrode 820. Particularly,the high frequency power source 819 comes to a result that it does notcontribute to causing plasma and because of this, it does not serve tosupply energy. However, even in this case, plasma processing can beperformed by applying a control voltage to the work electrode 109 fromthe work electrode voltage-controlling system 118 in the same manner asin the foregoing case and using energy to be supplied from the workelectrode voltage-controlling system 118 as the energy for causingplasma, thereby causing plasma 806.

In this case, if the magnitude of a voltage when the work electrodevoltage becomes negative is excessively small (about less than 100 V),no plasma is caused. Therefore, it is necessary to supply a relativelylarge electric power. In view of this, specific due regards should bemade with respect to energy control for ions and ion damage for a workto be treated.

EXPERIMENT 1 AND COMPARATIVE EXPERIMENT 1 EXPERIMENT 1

In order to observe the effectiveness of the foregoing apparatus, anexperiment was conducted for measuring ion energy impinged into a work.The measurement of the ion energy was performed using the ion energymeasuring means in the apparatus shown in FIG. 1(a) which was modifiedas shown in FIG. 4.

In FIG. 4, numeral reference 436 stands for a small aperture provided atthe work electrode 109, numeral reference 437 stands for an electronpreventive electrode applied with mesh-like treatment at the centerbeing connected to the work electrode 109, numeral reference 438 standsfor a control electrode which serves to restrict ion energy capable ofpassing through, numeral reference 439 stands for a DC power sourcecapable of varying the voltage which serves to vary said ion energy,numeral reference 442 stands for a DC power source for the electronpreventive electrode, numeral reference 440 stands for an ion collectingelectrode, and numeral reference 441 stands for an ammeter which servesto measure the current of an ion captured by said ion collectingelectrode. Other constitution than that mentioned above is the same asthat in the case of FIG. 1.

In the following, detailed explanation is to be made about the measuringmethod.

First, the vacuum vessel 103 was vacuum-evacuated to bring the inside toa vacuum of less than 10⁻⁶ Torr. Then, Ar gas was introduced through thegas inlet opening 105 at a flow rate of 20 sccm, and thevacuum-evacuated state was adjusted such that the pressure in the vacuumvessel 103 became 3×10⁻⁴ Torr. Successively, an electric current of 150A was flown to the air-cored coil 104 to produce a magnetic field of 875Gauss in the vacuum vessel 103, and microwave (2.45 GHz) of 200 W wassupplied through the microwave transmissive window 102, to thereby causeplasma 106.

As a specific measurement, voltage/current characteristics were observedwith respect to each of the langmuir probes 115a and 115b and anelectron temperature Te at each of the places was computed. As a resultof the computation, there were obtained the values: kTe-a=2.3 eV andkTe-b=2.5 eV. The variation for these value in terms of time passage waswithin the range of ±0.1 eV. Thus these value were not substantiallyvaried. In this experiment, there was employed a reduction ratio of 1/10for each of the attenuators 112a, 112b and 112c.

There were observed V_(T) -a and V_(T) -b at the position for each ofthe langmuir probes. As a result, there were obtained 11.9 V and 13.0 V.Each of the DC power sources 150a and 150b was adjusted to input 1.19 Vand 1.30 V into the plasma potential appreciating system 113.

In this experiment, a pulse generator 8112A available from HP Co., Ltd.was used as the reference waveform-outputting system 115, and therelated conditions were set to be t₁ =20 nsec, t₂ =1 μsec, V₁ 0.5 V, V₂5 V, 10 V and 15 V. As each of the DC power sources 131 and 132, therewere used those of 300 V and -300 V.

Ions and partial electrons arriving at the work electrode 109 passthrough the small aperture 436, the electrons are prevented by theelectron preventive electrode 437 applied with a negative voltage (-50V) through the work electrode 109, and only the ions pass through theelectron preventive electrode 437. And, the ions which pass through thecontrol electrode 438 and arrive at the ion collecting electrode 440 canbe controlled by applying a voltage Vi to the control electrode 438 fromthe DC power source 439 and varying said voltage. The energydistribution of ion was obtained by measuring the electric current Ii ofthe ion captured by the ion collecting electrode 440 using the ammeter441 to obtain Vi--Ii characteristics and computing the dIi/dVi.

The results obtained were shown in FIG. 5.

In the figure, the curves of numeral references 561, 62 and 563 are theenergy distributions of ion obtained respectively at the time of V₂ =5V, 10 V and 15 V.

It was recognized that each of these energy distributions comprises anenergy center of providing a maximum distribution and is of a singledistribution having a sharp peak; the energy of providing such maximizeddistribution is almost precisely 10 holds over the Vi, particularly, themultiplying number of the reciprocal of the reduction ratio for theattenuator 112a, 112b or 112c; and the distribution width of energy,namely, the half-amplitude width, is 10 eV, 12 eV or 13 eV.

From the above measured results, it was found that for the energy of ionto be impinged into the work electrode, its distribution is small and itcan be precisely set to a desired value by the V₂.

COMPARATIVE EXPERIMENT 1

With respect to the work electrode, similar measurement was alsoperformed by using a conventional apparatus of the constitution shown inFIG. 6 while applying a high frequency power.

In FIG. 6, numeral reference 619 stands for a frequency power source(13.56 MHz) of 500 W in outputting power which is capable of changingits outputting power, numeral reference 623 stands for a matching box,and the remaining numeral references other than these are of the samemeanings as those designated by the corresponding numeral references inFIG. 4.

Plasma was caused under the same conditions and in the same manner as inthe above. Thereafter, high frequency power of 240 W (13.56 MHz wasapplied, and the matching box 623 was adjusted so as to minimizereflected wave. The bias voltage of the work electrode at this time was-100 V. At this voltage, the average ion energy was equivalent to thatat the time of V₂ =10 V in the antecedent Experiment 1.

Hereupon, the energy distribution of ion impinged into the workelectrode 109 was observed in the same manner as in the antecedentexperiment.

Numeral reference 564 in FIG. 5 stands for the energy distribution ofion obtained in this comparative experiment. It was recognized that theenergy of ion is widely distributed between 0 to 200 eV.

As is apparent from the experimental results obtained in the above twoexperiments, it was confirmed that in comparison with the case of usingthe conventional apparatus in which a high frequency power was appliedto the work electrode, the energy distribution of ion arriving at thework electrode is markedly improved by practicing the present inventionand ions with a completed energy distribution can be obtained.

Experiment 2

Using the apparatus shown in FIG. 4, there was appreciated therelationship between the t₁ and the t₂ of the signal relating to thereference waveform outputted by the reference waveform signal-outputtingsystem 115 shown in FIG. 2.

In the same manner as in the foregoing Experiment 1, there was caused Arplasma, the energy distribution of ion impinged into the work electrode109 was obtained, and the relationship between the t₁ and the t₂ whereinsaid energy distribution becomes to be of a single distribution wasobtained at the respective times of t₁ 10, 20 and 30 nsec. The resultsobtained were shown in FIG. 7, wherein a broken line 770 stands for thelinear line expressed by the equal sign of the foregoing equation (2),and the slanting line portion stands for the region expressed by thesign of inequality.

In each case where the t₁ was as above mentioned above, a stable singledistribution was provided at the limit for the energy distribution to bea single distribution and at the t₂ being shorter than that in the rangeof the t₂ being such mentioned by numeral reference 771, 772 or 773.This exists substantially on the linear line 770. Thus, it was confirmedthat the foregoing equation (2) is effective.

EXAMPLE 1 AND COMPARATIVE EXAMPLE 1 Example 1

There was provided a specimen to be treated as the work by forming a 100Å thick thermally oxidized film on a p-type Si substrate and depositinga 4000 Å thick polycrystalline Si film doped with phosphorous on saidoxidized film.

The resultant was subjected to etching in the apparatus shown in FIG. 1to prepare a MOS structure.

That is, the above specimen was applied with a resist (available underthe trademark name of OFPR-800, produced by Tokyo Ohkakohgyo KabushikiKaisha), followed by subjecting to exposure and development treatment bya conventional exposure device in the semiconductor field, to therebyresist pattern on the specimen.

In order to observe the electron temperature of plasma, a dummy specimenidentical to the above specimen was placed on the work electrode 109,and the vacuum vessel 103 was vacuum-evacuated to bring its inside to apressure of less than 10⁻⁶ Torr. Then, Cl₂ gas was introduced throughthe gas inlet opening 105 at a flow rate of 20 sccm, and the innerpressure of the vacuum vessel was adjusted to 3×10⁻⁴ Torr. Electriccurrent of 150 A was flown to the air-cored coil 104 to generate amagnetic field of 875 Gauss in the vacuum vessel 103 and concurrently,microwave (2.45 GHz) of 200 W was supplied through the microwavetransmissive window 102, to thereby cause chlorine plasma 106. In thiscase, no voltage was applied from the work electrode voltage-controllingsystem 118.

Using the langmuir probes 111a and 111b, the voltage/currentcharacteristics were observed for each of the langmuir probes to obtainan electron temperature of the plasma at the respective positions whereeach of the langmuir probes were situated. As a result, the electrontemperature with respect to the langmuir probe 111a was kTe-a=2.5 eV,and the electron temperature with respect to the langmuir probe 111b waskTe-b=2.7 eV. The V_(T) of the foregoing equation (1) was obtained basedon these resultant values for the electron temperature. As a result, theV_(T) at each of those positions, that is, the V_(T) -a and V_(T) -bwere 12.8 eV and 13.9 eV respectively.

In this example, each of the attenuators 112a, 112b and 112c was set toa conversion rate of 1/10, and because of this, the outputting voltageof each of the setting DC power sources 150a and 150b was set to avoltage corresponding to 1/10 of the value of the V_(T) -a or V_(T) -b.

As for the parameters with respect to the time and voltage of thereference waveform signal outputted by the reference waveformsignal-outputting system 115, they were set to be t₁ =20 nsec, t₂ =1μsec, V₁ =0.5 V and V₂ =10 V respectively, in order to make the energyof ion to be impinged into the specimen to be 100 eV. And as the DCpower sources 131 and 132, there were used a DC power source of 300 V inoutputting voltage and a DC power source of -300 V in outputtingvoltage.

Next, the foregoing specimen 108 to be processed was placed on the workelectrode 109. Thereafter, the above procedures employed in the case ofthe dummy specimen were repeated to cause chlorine plasma 106. Acontrolling voltage from the work electrode voltage controlling system18 was applied to etch the polycrystalline Si with ions with completeenergy, whereby a MOS structure having a polycrystalline Si electrode of1 mm in diameter was prepared. The rate of etching the polycrystallineSi in this case was 6200 Å/min.

The resultant MOS structure was subjected to dielectric breakdown testby applying a voltage between its Si substrate and its polycrystallineSi electrode. As a result, the dielectric breakdown electric field ofthe MOS structure was 4 to 6 MV/cm.

COMPARATIVE EXAMPLE 1

Using an etching apparatus comprising the conventional apparatus shownin FIG. 6 from which the ion energy measuring mechanism situated underthe work electrode 109 being removed, the procedures of Example 1 wererepeated.

Specifically, a specimen of the same kind as in Example 1 was placed onthe work electrode 109, chlorine plasma was caused by way of microwaveenergy, and high frequency power of 340 W (13.56 MHz) from the highfrequency power source 619 was applied to the work electrode 109 toperform etching, whereby a MOS structure having a polycrystalline Sielectrode of 1 mm in diameter was prepared. At this time, the biasvoltage caused at the work electrode 109 was -100 V, and because ofthis, in average, the energy possessed by ion became the same as that inthe case of Example 1. The etching rate in this case was 3400 Å/min.

As well as in the case of Example 1, the resultant was subjected todielectric breakdown test by applying a voltage between its Si substrateand its polycrystalline Si electrode. As a result, the dielectricbreakdown electric field of the MOS structure was 1 to 3 MV/cm.

The followings was found from the the results mentioned above. That is,in the case of Example 1, ion energies can be completed to be in a statesuitable for etching in the range of not imparting ion damages to theSiO₂ film and because of this, the etching rate is increased and thebreakdown voltage of an element is improved in comparison withComparative Example 1.

In consequence, according to the plasma processing method and apparatusof the present invention, energy of ion can be completed to be in adesired state and because of this, desirable plasma processing can beperformed with an improved efficiency.

EXAMPLE 2 AND COMPARATIVE EXAMPLE 2 EXAMPLE 2

Using the apparatus shown in FIG. 8, a 2000 Å thick hydrogenatedamorphous silicon nitride film (a-SiN:H) was formed on a Si substrate.

That is, a dummy Si substrate 108 was placed on the work electrode 109being maintained at 300° C. by means of a heater (not shown in thefigure) installed in the work electrode 109. The dummy substrate washeated to and maintained at 300° C. The vacuum vessel 803 wasvacuum-evacuated to bring its inside to a pressure of less than 10⁻⁶Torr. Successively, SiH₄ gas, N₂ gas and H₂ gas were introduced throughthe gas inlet opening 805 at respective flow rates of 3 sccm, 100 sccmand 40 sccm, and the inner pressure of the vacuum vessel was adjusted to0.6 Torr.

As the high frequency power source 819, there was used a high frequencypower source of 300 W (100 MHz) in outputting power. The switch 821 waspositioned on the side of the high frequency power source 819. Then, ahigh frequency power was applied to the upper electrode 820, and thematching box was adjusted such that reflected wave became minimum. Inthis way, there was caused plasma 806 between the upper electrode 820and the work electrode 109. In this case, no voltage was applied fromthe work electrode voltage-controlling system 118.

In the same manner as in Example 1, there was observed an electrontemperature of the plasma at the respective positions where each of thelangmuir probes 111a and 111b were situated. As a result, the electrontemperature with respect to the langmuir probe 111a was kTe-a=3.3 eV,and the electron temperature with respect to the langmuir probe 111b waskTe-b=3.1 eV. The V_(T) of the foregoing equation (1) was obtained basedon these resultant values for the electron temperature. As a result, theV_(T) at each of those positions, that is, the V_(T) -a and V_(T) -bwere 16.6 eV and 15.6 eV respectively.

In this example, each of the attenuators 112a, 112b and 112c was set toa conversion rate of 1/10, and because of this, the outputting voltageof each of the setting DC power sources 150a an 150b was set to avoltage corresponding to 1/10 of the value of the V_(T) -a or V_(T) -b.

A for the parameters with respect to the time and voltage for thereference waveform signal outputted by the reference waveformsignal-outputting system 115, they were set to be t₁ =20 nsec, t₂ =0.8μsec, V₁ 0.3 V and V₂ =3 V respectively, in order to make the energy ofion to be impinged into the specimen to be 30 eV. And as the DC powersources 131 and 132, there were used a DC power source of 300 V inoutputting voltage and a DC power source of -300 V in outputtingvoltage.

Next, a Si substrate 108 as the work to be processed was placed on thework electrode 109. Thereafter, the above conditions and proceduresemployed in the case of the dummy specimen were repeated to cause plasma806.

A controlling voltage from the work electrode voltage-controlling system118 was applied, and a 2000 Å thick a-SiN:H film was formed on the Sisubstrate by way of a plasma CVD method using ions with a completeenergy. The deposition rate in this case was 14 nm/min. Thereafter, anAl electrode was formed on the resultant a-SiN:H film, followed bysubjecting to dielectric breakdown test. As a result, the dielectricbreakdown electric field of the resultant a-SiN:H film was 10 to 12MV/cm.

COMPARATIVE EXAMPLE 2

Using a plasma processing apparatus comprising the apparatus shown inFIG. 8 from which the work electrode voltage 118 and the attenuator 112being removed, and the work electrode 109 being grounded (that is, beingelectrically connected to the vacuum vessel), without using the langmuirprobes 111a and 111b, the procedures of Example 2 were repeated to forma 2000 Å thick a-SiN:H film on a Si substrate. The deposition rate inthis case was 11 nm/min. In the same manner as in Example 2 was formedon the resultant a-SiN:H film, followed by subjecting to dielectricbreakdown test. As a result, the dielectric breakdown electric field ofthe resultant a-SiN:H film was 5 to 7 MV/cm.

The following was found from the results mentioned above. That is, thea-SiN:H film deposited according to the method and the apparatus of thepresent invention is surpassing the a-SiN:H film deposited according tothe conventional method with respect to deposition rate and breakdownvoltage. According to the plasma processing method and apparatus,desirable plasma processing can be performed with an improvedefficiency.

I claim:
 1. A plasma processing method comprising introducing aprocessing gas into a vacuum vessel containing a work to be processedplaced on a work electrode, applying a plasma generating energy to saidprocessing gas to generate plasma and processing said work to beprocessed with said plasma, comprising the steps:repeating alternately afirst step and a second step; the first step comprising carrying out ionirradiation by applying a negative voltage to said work electrode whilecontrolling said voltage such that the voltage of said work electrodeversus the potential of said plasma is maintained constant, therebymaking the energy possessed by ion to be irradiated to said work to beprocessed to provide an energy dispersed state as desired; and thesecond step comprising irradiating electrons in said plasma to said workto be processed by applying a positive voltage to said work electrode.2. The plasma processing method according to claim 1, wherein the plasmagenerating energy is microwave energy or high frequency energy.
 3. Aplasma processing apparatus comprising a vacuum vessel having anelectrode on which a work to be processed is to be placed, said vacuumvessel confining plasma for performing plasma processing for said work;comprising:a gas supply means for supplying into said vacuum vessel aprocessing gas to be used for said plasma processing for said work; anexhaust means for vacuum-evacuating the inside of said vacuum vessel;and a voltage applying means for applying a voltage to said workelectrode and controlling ions in said plasma to be impinged into saidwork to be processed, said voltage applying means designed such that apositive voltage and a negative voltage are allowed to be alternatelyapplied to said work electrode, and the voltage of said work electrodeversus the potential of said plasma controlled to be constant when thevoltage of said work electrode versus the potential of said plasma isnegative.